1. Field of the Invention
The present invention relates generally to optical systems, and more specifically to suppressing stimulated Brillouin scattering in an optical system.
2. Discussion of the Related Art
In long distance optical fiber transmission systems, it is desirable to launch signaling using as high an optical power as possible, enabling the lightwave signals to be transmitted with fewer additional components such as repeaters and amplifiers, which increase the cost of communication systems. However, the combination of high powered, narrow linewidth optical sources with low-loss single mode transmission fiber opens the possibility of signal degradation and increased bit error rates, attributable to a host of nonlinear fiber-related phenomena. These nonlinear phenomena include stimulated Brillouin scattering (SBS), stimulated Raman scattering, and self-phase modulation. Similarly, in high power fiber laser systems, it is desirable to obtain as high a power as possible and to maintain narrow linewidth for high coherence. SBS is the primary power limiting mechanism for achieving high power with narrow linewidth due to its low threshold.
Brillouin scattering, which may be present within a fiber or other medium, results from photons being scattered by localized refractive index variations induced by acoustic waves. These refractive index variations are typically caused by thermal fluctuation in the medium. At the presence of a strong light field, the variation of the refractive index is enhanced, and more light is scattered. This process is commonly referred to as stimulated Brillouin scattering. Ultimately, light from an intense forward propagating signal (commonly referred to as a “pump” signal) can provide gain for a backward propagating SBS or “Stokes” wave or signal. This scenario is a classical description of SBS.
SBS threshold power may be defined as the power level of an input optical pump signal at which the power of the backward Stokes wave becomes equal to the power level of the input optical pump signal at the fiber input. SBS threshold power generally increases with the linewidth of the light being propagated along a medium. For this reason, concern over the adverse effects of SBS was minimal—until the introduction of narrow linewidth laser sources. As use of narrow linewidth sources increases, such lasers are likely to be the optical source of choice for future optical fiber transmission systems, among other types of systems. In such systems, SBS has the potential for significantly contributing to signal degradation at relatively low input power levels.
Some techniques to suppress the effects of SBS include broadening the laser linewidth via phase modulation, frequency dithering, and amplitude or phase noise addition. Examples of such techniques are disclosed in U.S. Pat. Nos. 7,146,110 and 6,166,837. In some systems, SBS suppression takes the form of polarization scrambling, such as that which is disclosed in U.S. Pat. Nos. 6,414,772 and 5,787,211. Other techniques include increasing the mode area and/or reducing the fiber length to increase the SBS threshold. In some cases, fiber manufactures attempt to reduce the strength of acoustic wave in order to suppress SBS. As another example, U.S. Pat. No. 6,8507,12 is directed toward an optical fiber transmission system which introduces incoherence between the polarization states in order to reduce SBS.